Portable electronic equipment such as cellular phones, personal digital assistances, notebook-sized personal computers, portable audios and portable visuals has been becoming popular. Such portable electronic equipment is conventionally driven by primary cells or secondary cells. While the primary cells are single-use cells, the secondary cells are reusable and require chargers and charging time since they need to be charged after consumption of a specific amount of electric power. As the secondary cells, NiCd batteries or lithium-ion batteries are used, and although small-size batteries with high-energy density have been developed, batteries supporting continuous driving for a longer period of time are being demanded.
In order to meet the demand, fuel cells operated without charging have been proposed. The fuel cells are generators for electrochemically converting chemical energy of fuel to electric energy. Among the fuel cells, a Polymer Electrolyte Fuel Cell (PEFC) for generating electric power with use of a perfluorocarbon sulfonic acid-based electrolyte to reduce hydrogen gas in an anode and to reduce oxygen in a cathode is known as a cell with high output density, and its development is being pursued for application to automobiles and the like.
However, in the PEFC, hydrogen gas, is low in volumetric energy density, which necessitates the volume of a fuel tank to be enlarged. Further, auxiliary equipment such as devices to feed fuel gas and oxide gas to a power generator in the PEFC as well as humidifiers for stabilizing battery performance are required, which increases the size of the PEFC, making the PEFC unsuitable as a power source for portable electronic equipment. Accordingly, a Direct Methanol Fuel Cell (DMFC) for generating electric power by directly extracting protons from methanol is now under development.
The DMFC, although having a disadvantage that its output is smaller than that of the PEFC, has advantages that the volumetric energy density of fuel can be increased and auxiliary equipment for the power generator can be reduced, which allows downsizing. Because of this reason, the DMFC is drawing attention as a power source for portable electronic equipment and several proposals have been made.
The DMFC performs power generation by the reaction as defined in the following equation:Anode side: CH3OH+H2O→6H++6e−+CO2Cathode side: 6H++6e−+3/2O2→3H2O
More specifically, in the anode, methanol and water react by catalysis of a catalyst including ruthenium and platinum to produce hydrogen ions, electrons and carbon dioxide, where the electrons are outputted outside as electric power from the anode and the hydrogen ions are transmitted to the cathode side through an ion-permeable membrane. In the cathode, hydrogen ions receive electrons from the cathode and react with oxygen in the air to produce water. In this case, the efficiency of the DMFC is high when a power generation section is at a high temperature as high as the characteristics of the ion-exchange membrane permit, i.e., in the range of 60° C. to 80° C., and in high humidity.
In the present DMFC, however, resistances due to various losses (polarization) generated inside the fuel cell hinder obtainment of a theoretical electromotive force that is an ideal output, resulting in an output lower than the theoretical electromotive force. The losses due to the polarization include a loss caused by resistances called resistance polarization which hinders the flow of ions and electrons in the electrolyte, a loss caused by activation polarization due to consumption of activation energy in an electrochemical reaction, and a loss caused by so-called diffusion polarization due to consumption which occurs when reactants and reaction products spontaneously diffuse and migrate due to a continuous chemical reaction.
These polarizations occur in the anode and the cathode, and material development for solving these problems are under way. Though some improvement is seen, the present state is that if, for example, an ideal electromotive force of the DMFC is 1.2V and a theoretical efficiency is 97%, an electromotive force obtained in a practical level is at most around 0.3V due to internal voltage drop.
Consequently, the V-I characteristic of an output voltage of the fuel cell itself against a load current has a drooping characteristic with depending largely on load current, whereas output voltages of other secondary cell and primary cell are almost constant and stabilized with respect to the load current. More specifically, the fuel cell has a characteristic that extraction of a large amount of load current lowers the output voltage because of resistance inside the fuel cell. Therefore, the fuel cell generally has an optimum current which allows extraction of a maximum amount of electric power.
Moreover, the characteristic of the fuel cell implies that a method with use of auxiliary equipment such as pumps for feeding fuel and circulating air is practical and allows a stable operation and an end operation. However, this method entails a drawback that power generation is not started unless air and fuel are supplied to a battery cell of the fuel cell during start-up operation. Once the power generation is started, feeding of electric power to the auxiliary equipment such as pumps becomes possible with self generated electric power, though other auxiliary power sources, typically chargeable secondary cells, are necessary during the start-up operation or the end operation.
Examples of parallel operation of a secondary power source and a fuel cell have been disclosed in Patent Documents 1 and 2 shown below, both of which aim at stable feeding of electric power and therefore do not focus on the constitution for maximizing the power generation capacity of the fuel cell.
Patent Document 1: Japanese unexamined patent application No. 59-230434, and
Patent Document 2: Japanese unexamined patent application No. 3-40729